The present invention relates to the digital color image processing arts. It finds particular application in conjunction with generating a black component for output in CMYK color space, and will be described with particular reference thereto. However, it is to be appreciated that the invention is applicable to various transforms between input and output color spaces.
Color in printed digital images results from the combination of a limited set of colors over a small area in densities selected to integrate the desired color response. This is accomplished in many printing devices by reproducing so called “separations” of the image, where each separation provides varying values of a single primary color. When the separations are combined together, the result is a full color image.
The particular color of each separation depends on the “color space” being implemented. Examples of color space models include, RGB, CMY, CMYK, Lab, Yes, YIQ, HSV, HLS.
In practice, color images are commonly printed in a cyan-magenta-yellow-black (CMYK) color-space. This color space is based upon the CMY color-space, but attempts to improve the quality of “black” in the image and reduce use of color inks, or toners, or dies, or the like. In theory, images can be printed using the CMY color space, with a mixture of the three colors producing black. In practice, however, printing with only cyan, magenta, and yellow inks often does not produce the highest quality black, but instead results in a muddy brownish output due to impurities in the inks, the particular paper or other image recording media used, and the partial reflection of light instead of its complete absorption into the inks. Furthermore, select use of black ink in place of the primary colors reduces expense and minimizes the total amount of ink used which is often desirable in ink-jet and other printing applications where the ability of the recording substrate to absorb ink is limited.
Methods for converting to the CMYK color space include those referred to as “under color removal” (UCR) and “gray component replacement” (GCR). UCR/GCR methods vary, but commonly involve examining the individual pixels of an image using the lowest or “darkest” of the three cyan-magenta-yellow colors to determine an amount of black to be added (Gray Component Replacement). One or more of the CMY colors are then adjusted to account for the addition of black ink (Under Color Removal). For example, if a given pixel of an image is represented in the CMY color space by C=0.5, M=0.4, and Y=0.25, then the black or K value would be based upon the lowest or Y value. In a 50% gray component replacement (GCR) method, K=(50% of Y)=0.125. In a typical under color removal (UCR) step, the remaining CMY values would then each be reduced by 0.125 so that the resulting UCR/GCR pixel is represented by C=0.375, M=0.275, Y=0.125, and K=0.125. Of course, other UCR/GCR methods are known, but each seeks to determine the level of black for a given pixel, based only on the intermediate color space, e.g. CMY.
The present invention contemplates a new method and apparatus to generate output color components that yield increased output quality.